The long-term objective of our research is to better understand the mechanisms underlying both normal and abnormal ocular motor and vestibular function in Huntington's Disease (HD). Our recent research has shown that patients with HD have selective deficits in generating "volitional" saccades (e.g. antisaccades and predictive saccades) as well as defects in suppression of unwanted reflexive saccades in response to novel stimuli. The first aim of the present proposal is to develop more subtle ways of testing and quantifying the higher level control of saccadic and also pursuit eye movements. A new saccade paradigm will be tested that measures the amplitude and latency of saccades while making sequences of saccades to a series of seen or remembered targets. We will also compare the gain and timing of smooth pursuit movements to stimuli that produce a more reflexive response (sudden movement of a full field stimulus) with stimuli that produce a more volitional response (tracking of a small target moving on a visually rich background). We expect HD patients to show deficits in more volitional pursuit. The second aim is to evaluate further postural and vestibular control in patients with HD. Using moving platform posturography, we will measure postural control in patients with HD and compare the results with measurement of their vestibulo-ocular reflex. We will test the hypothesis that the ataxia of HD patients is related to a disorder of central processing of vestibular information. All of our testing results will be correlated with results of tests of neurological and psychological function and MRI studies.